Aircraft flight operations may involve patterns that occur in close proximity to the ground. Helicopter flight operations, in particular, typically involve a significant amount of flight time in close proximity to the ground. This is particularly true of military helicopters with the advent of more sophisticated avionics and flight equipment that make nap-of-the-earth flight operations feasible as a routine tactical flight philosophy.
Helicopter flight operations in close proximity to the ground are subject to two serious types of ground threats: ground fire (active); and wire/cable strikes (passive). These ground threats have become a serious concern with the increasing emphasis on nap-of-the-earth helicopter flight operations.
During the Vietnam war, many helicopters were neutralized or destroyed as a result of encounters with steel cables stretched between trees. Such cables were purposely positioned to rap around the helicopter rotor head to disable the helicopter, or, if the speed of the helicopter was sufficiently high, to tear through the helicopter fuselage for complete destruction thereof. To mitigate this liability at low level helicopter flight operations, wire cutter systems were developed and incorporated into the basic helicopter configuration. Such wire cutter systems are operative to deflect wires/cables encountered during low level flight operations into the throat of a wire cutter. The momentum of the helicopter, in combination with the sharpness of the cutter blades, is sufficient to sever a wire/cable before damage can be inflicted upon the helicopter.
Wire cutter systems are currently utilized on helicopters engaged in low level flight operations where there is a high probability of encountering wires/cables. For example, almost all military helicopters incorporate wire cutter systems. In addition, federal and state helicopters utilized in illegal alien operations typically incorporate wire cutter systems. It is not uncommon for such helicopters to strike numerous telephone and/or power lines while engaged in nap-of-the-earth flight operations. Telephone and/or power lines in themselves are not the primary concerns, but rather the steel cables (up to 3/8 in. diameter) that are disposed in combination with such lines for the support thereof. These steel cables are similar to the type encountered during the Vietnam war.
Although wire cutter systems provide increased survivability in the event of a wire/cable strike, difficulty arises when low observability is required in a helicopter design. The wire cutter system cutting jaws (blades) are primarily composed of steel which reflects microwave energy, and therefore increase the radar signature of the aircraft. Additionally, wire cutters are typically mounted above or in front of the basic aerodynamic fairings on the helicopter. Therefore, the aerodynamic shape of the aircraft may be altered, further increasing the aircraft's radar signature. It has also been found that wire cutter blades manufactured of composite materials to reduce the returned radar energy are not a viable alternative to steel because of strength requirements. Another problem that must be dealt with by current wire cutter systems is the ability to sever slack wire and cables, such as those which unwrap after the launch of a missile, e.g., a TOW missile. Such slack wires/cables may cause serious damage to a helicopter by becoming entangled in the main rotor push rods, or in the main rotor itself.